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Climate is the average state of the weather. And weather is concerned
with daily changes in temperature, wind, cloud and rain in general
the state of the atmosphere. When we examine the climate of Jamaica we
are chiefly concerned with:
- Temperature (degree of heat and cold)
- Winds (their movement and direction)
- Rainfall (its causes and its seasons).
TEMPERATURE
How hot or cold a place is depends mainly on how far
it is north or south of the equator. The higher the latitude the colder
the climate. What is the latitude of Jamaica? If you look at the globe
you will see that Jamaica is in the tropical zone south of the Tropic of
Cancer. However, its distance north of the equator has a moderating effect
on its temperature, and hence Jamaica is said to have a semi-tropical climate.
Apart from latitude, the greatest factor in determining the temperature
of a country is altitude. Most of the effective heat we enjoy is radiated
from the earth which has been warmed by the sun. In reaching the surface
of the earth the sun's rays have to pass through layers of atmosphere,
a process which causes it to lose some of its heat. If we imagine these
layers of atmosphere as blankets retaining the heat radiated by the earth's
surface we will see that by climbing above these layers it will become
colder. Temperature decreases by 1.7 degrees Celsius (or 1 degree Fahrenheit)
for every 100 metres (300 ft) of ascent.
Because Jamaica is a very mountainous country, temperatures vary widely
in different parts of the island. For example, the temperature might drop
to about 10 degrees C or 50 degrees F in Mandeville, 626 m (2,061 ft),
while in Kingston the mean temperature is 26 degrees C or 78 degrees F.
On the whole, Jamaica's climate has no extremes, especially since the surrounding
sea has a moderating effect on the weather, and the variety of climate
is considered healthy and beneficial.
WINDS
Local winds, from the sea by day and from the land at night,
are very noticeable in Jamaica because it is an island. The prevailing
winds in the West Indies are from the north-east. Remember that warm air
is lighter than cold air, and that winds blow from an area of high pressure
to an area of low pressure. At the equator, the hot air is continually
rising, creating an area of low pressure. The air on both sides of the
equator, being cooler and heavier, moves in to take the place of the rising
air, hence there is a constant movement of air towards the equator, from
the north and south. However, since the earth turns from west to east,
the winds do not blow due south or north, but are slightly deflected so
that they come from a north-east direction north of the equator, and from
a south-east direction south of the equator. These movements of air are
called the Trade Winds. Because they are blowing from a cooler to a warmer
part of the earth, they are able to hold more moisture.
RAINFALL
The heat of the sun, like the fire under a kettle, turns
water from the various water bodies of the world into water vapour. This
process is called evaporation. If the temperature of air is lowered, the
water vapour it contains in the form of clouds will fall as rain. This
process is called condensation. Jamaica receives two kinds of rainfall:
convectional and relief. It does not receive monsoon rains because it is
not a large land mass.
Moisture-laden air may be cooled and thus forced to give up its water vapour
in the form of rain by 1) rising to colder regions of the atmosphere or
2) being forced upwards by land masses in its path. In both cases condensation
takes place and rain falls.
In the first case, when evaporation takes place, the moist air rises, cools
and condenses and rain falls. Rains originating in this way are known as
convectional rains. When the moisture-bearing trade winds come upon
a high mountain range, they are forced to rise; condensation takes place,
and the rain which results is known as relief rainfall because
it is caused by a change in altitude. The wind deposits its moisture on
the windward side of the mountains, and then descends as a dry wind on
the leeward side of the mountains. This dry area is known as the rain shadow.
Relief rainfall causes Port Antonio on the windward side of the Blue Mountains
to receive an average of 430 mm (17 inches) of rain in November while Kingston,
in the rain shadow, receives only 175 mm (7 inches). Antigua and Barbados,
which are two flat islands, have a lower rainfall than the more mountainous
Caribbean islands.
As a rule, rainfall is much heavier on the North Coast of Jamaica, which
receives the relief rainfall provided by the mountains running from west
to east, than on the south coast, which receives chiefly convectional rain.
Jamaica has two rainy seasons, the first in May and the other in October
and November. As a rule, rain follows the sun. It appears to pass over
all places between the Tropics of Cancer and Capricorn twice a year. In
Jamaica the sun is directly overhead about the second week in May and in
early August. Notice that the periods of heavy rainfall reach their maximum
shortly after the sun has been directly overhead.
HURRICANES
A hurricane is a storm revolving around a centre of low pressure
which contains almost no wind. As a hurricane develops, the winds from
the area of high pressure rush towards the low-pressure centre, and as
their centrifugal force intensifies, powerful gales of up to 200 kilometres
(120 miles) per hour are built up. The calm vortex in the centre, the 'eye'
of the hurricane, varies in diameter from thirty to a few hundred miles,
and usually moves westwards. Hurricanes are invariably accompanied by driving
rains.
Nowadays weather stations discover hurricanes as soon as they develop and
give warning to those places towards which a hurricane may be heading.
On the approach of a hurricane the barometer falls and the thermometer
usually rises.
The hurricane season is between July and October, though a hurricane may
occasionally arise in June or November. On September 12, 1988, Jamaica
suffered the worst hurricane in living memory when Hurricane Gilbert devastated
the island.
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